Direct numerical simulation of forced circular jets: Effect of varicose perturbation

Free circular jets at low and intermediate Reynolds numbers evolve through discrete transitions accompanied by the formation of large-scale structures. Understanding dynamics of coherent structures in free jets suggests control strategies that can be used to achieve enhanced spreading and mixing of the jet with the surrounding fluid. In the present work, an active control technique that alters the vortex dynamics in the jet is implemented close to the nozzle resulting in an effect over the entire flow field. Among techniques available for control, the one based on (axisymmetric) varicose perturbation has been considered. The effect of perturbation on jet evolution and flow structures is examined via a computational approach. Direct numerical simulation (DNS) of incompressible, spatially developing circular jets is reported for a Reynolds number of 1000. The three-dimensional unsteady Navier-Stokes equations are solved by a high order spatial and temporal discretization scheme. For small-scale perturbation using Gaussian white noise, the jet undergoes transition in conformity with experiments. With varicose perturbation, excitation frequencies play a significant role in the evolution of circular jets. Excitation frequencies of 0.1 and 0.2 show the formation of a trailing jet. For a frequency of 0.2, the circular jet shows pairing of the shed vortices. With further increase in the excitation frequency, formation of strong secondary hairpin vortices at the far field is to be seen. Largest influence of excitation is seen for frequencies around the preferred mode value of 0.32. For A thick shear layer helps in formation of vortex ring that undergoes tearing while a thinner shear layer develops a stronger vortex ring.